Hi Kelzee!
My teen isn't working towards exams but almost everyone else we know is, so I have a reasonable idea how it works. I'll give my take on it and no doubt others will be along soon to tell you more.
Do you structure the day as per a school timetable? Totally up to you. We don't.
How many GCSEs can they take? Most GCSEs are impractical because it is difficult and expensive to get coursework validated. I think maths is the only exception. HE kids do IGCSEs instead because those are assessed purely on exam results. They are totally equivalent and universally recognised.
They can do as many or as few IGCSEs as they want, at whatever age they want, in any order. HE kids often take fewer than schoolchildren, typically just enough to get on to the next level of education. For instance if the college course they want requires five (I)GCSE passes grades A-C then they may not bother doing more. Doesn't mean they won't learn other subjects, they may just not sit the exams for them.
What about practical work i.e science? IIRC there are a very few IGCSEs which do require supervised practical work (drama?) and which are therefore pretty much impossible to do outside of school. I think the sciences are all OK though. You can do practical work yourself in order to help with learning if you can get hold of suitable equipment. But you don't have to have it validated and the mark is all down to the exam.
What are the best companies to buy courses from? Don't know, sorry!
Are tutors recommended? Some people use them, some don't. Some like distance learning programmes. Some enjoy home ed tutor-led study groups with, say, one tutor and six or eight home ed kids all working together toward a particular exam.
Do you adhere to school holidays? Totally up to you. We don't.
Is flexy school worth considering? Yes, but it is very difficult to get headteachers to agree, so don't get your hopes too high!
What if both parents work?(if parents cant be by the childs side at all times how do you keep them motivated) I doubt it's any harder than motivating teens who go to school to do revision: parents aren't usually standing at their side to make them do their work either. HE kids tend to be more engaged with their learning, because it's possible for them to be involved in all the decisions which affect them. For example, they don't have to do subjects which you and they don't find worthwhile. They don't have to sit through classes in which teachers are explaining things which they already know, or which are over their head. They can choose which revision materials to use.
Having said that, if both parents work full-time then that is a lot of hours for your daughter to spend on her own and she may get lonely or bored. I know I would, even though I like my own company! It would be good to find ways of getting her out of the house and doing a variety of things during that time.